If you inter-connect two '163 type counters in the manner shown here, then the maximum count is drastically reduced but you gain full control of the output mark-space ratio. This allows you a choice of harmonic spectra, perhaps to eliminate the 3rd harmonic, to maximise the 2nd harmonic or to effect a "digital tone control" between different organ "voices" with a fixed pitch, for example divide-by 16 implemented as 9:7, 10:6, 11:5, 12:4, 13:3, or 14:2.

The mark and space periods of the output pulse are individually set from 2 to 17 clock periods, allowing a rectangular pulse as simple as 2:3 or as obscure as (say) 13:17 (or its inverse).

BRIEF CONSTRUCTIONAL DETAILS

IC1 has 4 inputs, pulled HIGH by a SIL resistor pack (which can be found very cheaply on the surplus market).
(Note that this pullup is not shown on the general cct diagram).
When built on matrix strip-board, many of the tracks underneath the ICs are to be left intact. For IC1:

In this "virgin" state, output Y is a divide-by-34 square-wave (17:17) and there must be no other inputs to A-D.
(Note: W is not intended as an off-board [audio] output).

BRIEF OPERATIONAL DETAILS

Cutting any of the tracks between IC1 inputs A-D and outputs Qa-Qd symmetrically reduces both the mark and space periods.
With all 4 such tracks cut, output Y is reduced to a 2:2 square-wave, but you are now free to add jumpers to inputs A-D in order to achieve a rectangular pulse. These inputs may be connected high, low, to system output "Y" or to "inverting output W".
(Computing the links required first, as in the worked examples below, can save you cutting the tracks unnecessarily, then having to add extra links to ground).

The algorithm to compute the links (below) uses the 17’s complement of each period. The result for the Mark period is tabulated above the result for the Space period.

If the result has a "1" for an input in both lines, then that input is left pulled high by the resistor pack, after the track to Qn is cut.
If the result has a "0" for an input in both lines, then that input is either linked to ground or is left connected to its own output by the copper strip-board track.

If the result demands a "1" for an input for the Mark period but a "0" for the Space period, then that input is linked to system output Y. If the opposite is the case, then that input is linked to pseudo-inverting output W.

FURTHER CONSIDERATIONS

Swapping the mark-space values is the same as pulse inversion and makes no acoustic difference. This is exploited in the "digital tone control" linking requirements (see table below)

In general, it is the more complex ratios that are of aural interest: simpler ratios will be missing harmonics. For example, ratio 1∶3 might not be aurally satisfactory, as it has no 4th., 8th. or 12th. harmonic.

Naturally, output Y cannot be followed by another divider, else the duty-cycle will be lost. To preserve the duty-cycle when a sub-octave is required, IC1’s enables (pins 7 & 10) are used with the CARRY output from a parallel (synchronised) 1-in-n divider. For example, to switch the output between second and third sub-octave, IC1’s enables would be switched from a 1-in-4 to a 1-in-8 positive-going pulse.

Divide-by-16 tone select.gif

Description:

How "inverting" the mark-space might simplify the logic required to implement a 3-way tone-selector.

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Two worked link calculations.gif

Description:

Worked examples, computing the links required for 5:11 and 7:9.Qa, Qc indicate an intact copper strip, but could equally be replaced by links to Ground.

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5-11 Mark-Space jumpers.gif

Description:

The prototype used two old-type 1mm jumper-leads, cut in half. The four bare ends were soldered to the 4 input tracks. The 1mm plug ends were "jumped" into 1mm sockets connected to Gnd, Y and W. No jumpers to +5V are required because of the pul

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Mark-Space Generator, general.gif

Description:

General configuration, not showing pull-up pack nor any connections to A-D.With pins 7 & 10 tied directly high, octave control is not possible.

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